Illumination systems that contain blackbody light sources such as arc lamp sources or incandescent sources are usually designed so that no light is reflected or recycled back to the source. Blackbody light sources are excellent light absorbers and poor light reflectors. Any emitted light that does get back to the source is absorbed and lost, lowering the overall efficiency of the illumination system.
Certain types of light sources, such as some fluorescent light sources and some light emitting diodes (LEDs), can reflect light as well as emit light. Reflecting light sources can be used in illumination systems that recycle light back to the source. Recycled light that is returned to the source and that is subsequently reflected by the source can increase the effective brightness of the source. In addition, light sources that can reflect light instead of absorbing light can reduce absorption losses and increase the overall output efficiency of illumination systems.
The technical term brightness can be defined either in radiometric units or photometric units. In the radiometric system of units, the unit of light flux or radiant flux is expressed in watts and the unit for brightness is called radiance, which is defined as watts per square meter per steradian (where steradian is the unit of solid angle). The human eye, however, is more sensitive to some wavelengths of light (for example, green light) than it is to other wavelengths (for example, blue or red light). The photometric system is designed to take the human eye response into account and therefore brightness in the photometric system is brightness as observed by the human eye. In the photometric system, the unit of light flux as perceived by the human eye is called luminous flux and is expressed in units of lumens. The unit for brightness is called luminance, which is defined as lumens per square meter per steradian. The human eye is only sensitive to light in the wavelength range from approximately 400 nanometers to approximately 700 nanometers. Light having wavelengths less than about 400 nanometers or greater than about 700 nanometers has zero luminance, irrespective of the radiance values.
U.S. Pat. No. 6,869,206, U.S. Pat. No. 6,960,872 and to U.S. Pat. No. 7,040,774 describe light recycling systems that include light recycling cavities or envelopes that enclose one or more light reflecting LEDs. The light reflecting cavities or envelopes reflect and recycle a portion of the light emitted by the LEDs back to the LEDs. The light recycling cavity or envelope has an output aperture with an area that is smaller than the total emitting area of the enclosed LEDs. In such cases, it is possible for the light exiting the cavity or envelope to be brighter than an equivalent LED measured in the absence of recycling.
The three aforementioned applications disclose light recycling illumination systems that have substantially Lambertian light outputs. The light output distributions of these illumination systems generally extend from approximately −90 degrees to approximately +90 degrees. However, the three aforementioned applications do not disclose optical elements that both recycle light and restrict the angular range of the light output.
In this specification, angular extent is defined by the maximum emitting angles of the source. A planar Lambertian source, for example, emits light of constant brightness from −90 degrees to +90 degrees, where the angle is measured from a line perpendicular to the source. The angular extent of a planar Lambertian source is therefore −90 degrees to +90 degrees.
The angular range is defined in this specification as the angular spread between the points on the light output distribution where the light flux per steradian is one half of the peak flux per steradian. For a Lambertian distribution, the light flux per steradian is one-half of the peak value at −60 degrees and at +60 degrees. For a Lambertian source, the angular range is 120 degrees.
U.S. patent application Ser. No. 10/952,112 entitled “LIGHT EMITTING DIODES EXHIBITING BOTH HIGH REFLECTIVITY AND HIGH LIGHT EXTRACTION,” U.S. patent application Ser. No. 10/977,923 entitled “HIGH BRIGHTNESS LIGHT EMITTING DIODE LIGHT SOURCE” and U.S. patent application Ser. No. 10/952,229 entitled “LIGHT RECYCLING ILLUMINATION SYSTEMS UTILIZING LIGHT EMITTING DIODES,” disclose illuminations systems that include reflective polarizers or wavelength conversion layers that recycle light. However, the reflective polarizers or wavelength conversion layers do not restrict the angular range of the light output of the illumination systems.
In designing complex optical systems such as projection displays, it is important to try to match the angular light output of the source to the maximum acceptance angles of the remainder of the optical system. For example, some imaging light modulators for projection displays have areas ranging from approximately 150 square millimeters to approximately 520 square millimeters. The imaging light modulators can accept light only for angles between −12 degrees and +12 degrees, for example. For such imaging systems, optimizing the quantity called etendue is important.
When measured in air, a simplified equation for etendue is the product of the area of the light beam times the projected solid angle (measured in steradians) of the light beam. Equation 1 expresses the simplified etendue relationship for an imaging system.Etendue=(A)(Ω)  [Equation 1]The quantity A is the area of the light beam and Ω is the projected solid angle of the light beam. For planar sources, the quantity Ω can be expressed asΩ=π sin2(half-angle).  [Equation 2]The half-angle is one half of the full angle of the light beam. A light beam that has a full angle of 24 degrees (from −12 degrees to +12 degrees) has a half-angle of 12 degrees.
An imaging light modulator that has an area of 250 square millimeters and an acceptance angle of −12 degrees to +12 degrees, for example, has an etendue of approximately 34 mm2-steradians. To effectively utilize the light emitted by the light source, the etendue of the light source for this example should also be approximately 34 mm2-steradians or less. If the output from the light source is Lambertian and extends from −90 degrees to +90 degrees with a range of 120 degrees, the area of the light source should be approximately 11 square millimeters in order for the source to have the same etendue as the imaging light modulator. It is difficult for an LED-based illumination system to have such a small output area and still have sufficient output flux for a large projection display. If the light source output can be restricted to a smaller angular range, however, the source area can be made correspondingly larger.
It would be desirable to design LED light recycling illumination systems that incorporate optical elements that both recycle light and restrict the angular range of the light output. Such systems can have increased output brightness and efficiency compared to systems that do not recycle light. In addition, such systems reduce the etendue of the illumination system output in order to better match the etendue of other optical elements in more complex optical systems such as projection displays.